HIV-1 has long been recognized as the etiological agent in acquired immunodeficiency syndrome (AIDS). Although neoplasms arise in HIV-1-infected patients more frequently than in other forms of immunosuppression, the role of HIV-1 as an oncogenic virus has not yet been clarified. HIV-1 encodes for the Tat transcription protein, which is essential for efficient viral replication. Tat is a likely candidate to contribute to tumor pathogenesis in HIV-1-infected patients because of its growth promoting activity, angiogenic function and regulation of the apoptotic pathway. The oncogenic role of Tat is further supported by an increased incidence of tumors in Tat-transgenic mice. 2-12 percent of AIDS patients develop primary central nervous system (CNS) lymphomas, of which 98 percent are B-cell lymphomas. Recently, a virus-linked mechanism of lymphomagenesis involving the Rb2/pl30 pathway has been proposed in AIDS-related Burkitt's lymphoma (BL). A deregulation of cell growth control by RB-related proteins may be the first step in lymphomagenesis in HIV-1-infected patients. However, little is known about the mechanisms by which HIV-1 gene products interact with the RB family and other cell cycle regulatory proteins. Among the latter, Cdk9 (PITALRE), identified and cloned in our laboratory, is the most likely candidate to be involved in the development of AIDS-related neoplasms. Cdk9, a cdc2-related kinase, promotes general elongation of transcription by activating the C-Terminal Domain (CTD) of RNA Polymerase II. Cdk9 is a partner of cyclin T1, which binds to the HIV Tat protein, supporting the positive involvement of Cdk9 in HIV replication and suggesting its possible role in the development of AIDS-related neoplasms. The goal of this proposal is to investigate the interaction between HIV gene products and cell cycle regulatory proteins and their role in the development/growth of the most common AIDS-related CNS and other subtype lymphomas. In particular, we will focus our attention on the cell cycle regulatory proteins, pRb2/pl30, Cdk9 and cycT1, which seem to be involved in AIDS-related tumorigenesis.